Historically, multi-car elevator control systems have used a separate controller to handle dispatching of hall calls to individual cars. Information is exchanged between cars and the group controller over serial communication links, using any number of communications methodologies, such as asynchronous or synchronous data transfer, and either full duplex or half-duplex communications lines. Should the communications link between the controller and any individual car be lost, the car could enter an automatic dispatching mode which would allow hall calls to continue to be serviced. Thus, the group controller would re-assign hall calls to another car with which the group controller still has contact. The car which lost contact with the group could enter an emergency dispatching mode of operation where it would answer all hall calls. The group controller also provided an ideal access point for elevator performance monitoring equipment because the group controller was the central receiving point for information relating the operations of all the cars in the group, and because the group controller was rarely taken out of service for maintenance or troubleshooting in comparison to individual cars.
Advancements in microprocessor technology allow the group control function to be integrated into each car controller. In such systems, each car controller is assigned a priority within the group, and the car with the highest priority controls the group function. Such a system is desirable because it eliminates the need for a separate group controller, reducing the cost of the elevator control system, and because it provides a built-in redundancy, each car controller being capable of assuming group control operations in the event of a failure of the group function in the primary car controller.
Whether there is a separate group controller or group control is performed within one of the car controllers, a failure on the serial communications line between cars can interfere with, or cause a loss of, group function. If one or more cars are cut off from the primary group controller, elevator operation will be degraded, because the group controller has fewer cars available to which it can assign incoming hall calls.
A communications failure also prevents elevator performance monitoring equipment from communicating with car controllers upstream of the failure point.
Additionally, in the case of systems in which each car has a group control capability, the stranded car, or one of the stranded cars (if more than one is cut off from the primary group controller), will assume the group function. As a result, when a hall call is generated, two cars (one assigned by the primary group controller and one from the stranded group of cars) will end up chasing the same hall call. This further degrades group operation and wastes energy.